1. Field of the Invention
The invention relates to a rotatable knob for the fixation of an apparatus support.
2. Description of the Prior Art
FIG. 1 shows a known rotatable knob for the fixation of an apparatus support in a housing by means of a clamping device having a spindle. The rotatable knob is mounted on the housing and also on the spindle. The rotatable knob comprises a knob shell 1 having an inner thread, a coupling part 2 for connecting the knob shell 1 to the spindle, a sleeve 3 which can be connected to the housing, a cover disc 4 which is secured to the housing by the sleeve 3, a screw 5 for setting the holding force, the screw being arranged in the knob shell 1 and a clamping disc 6 which has an outer thread standing in engagement with the inner thread of the knob shell and which is guided on the sleeve 3 in such a way that the clamping disc 6 executes a relative movement with respect to the knob shell 1 by rotation of the knob shell 1.
For the fixation of the apparatus support the knob shell 1 is turned in the clock-wise sense. In this way the clamping device is actuated by the spindle and the clamping disc 6 is displaced axially until the apparatus support is fixed. By screwing in the screw 5 until it contacts the clamping disc 6 the position of the clamping disc which corresponds to the required holding force for the apparatus support is set. If apparatuses are used for which different holding forces are required the position is in each case set anew. With renewed fixing of the apparatus support the clamping disc is displaced until it contacts the screw 5 and a rotation of the knob shell 1 is thereby prevented. Thus the apparatus support is fixed.
When using the rotatable knob it has turned out that with incorrect handling, in particular with an excessive use of force the knob shell 1 can be turned further beyond the position corresponding to the holding force which has been set. In this way the clamping disc 6 is pressed against the screw 5 and jams in the thread turns. In this way the knob shell 1 and the clamping disc 6 are mutually blocked and indeed in such a way that the rotatable knob can no longer be turned. The consequence resulting from this is that the rotatable knob has to be completely replaced. Moreover the clamping action at the apparatus support can be influenced disadvantageously so that the apparatus support blocks or is not correctly fixed.
The invention is based on the object of improving a rotatable knob in this respect so that a problem-free fixation and release of the apparatus support is ensured.